Heat treatment method for accelerating precipitation of nanoscale carbides in w-containing alloy steel

ABSTRACT

A heat treatment method for accelerating precipitation of nanoscale carbides in a W-containing alloy steel, including: homogenizing a billet at 1100-1200° C. for 24-48 h followed by furnace cooling to room temperature; then austenitizing the billet at 850-1000° C. for 20-40 min followed by quenching in iced brine; heating the billet to 650-750° C. with a rate of 3-7° C./min under a vacuum of 10 −3 -10 −2  Pa and a magnetic field of 10-14 T, isothermalizing the billet for 0.5-2.5 h and cooling the billet to room temperature. The chemical composition of the billet is: 0.06-0.14 wt % C, 1.50-3.00 wt % W, &lt;0.01 wt % P, &lt;0.005 wt % S, Fe and an inevitable impurity. A temperature of the iced brine is −3 to −1° C. The invention has the advantages of simple process, low cost and shortened production period. The W-containing alloy steel treated by the method has improved strength.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from Chinese Patent Application No. 201810606569.0, filed on Jun. 13, 2018. The content of the aforementioned application, including any intervening amendments thereto, is incorporated herein with reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to precipitation of carbides in W-containing alloy steel, and more specifically to a heat treatment method for accelerating precipitation of nanoscale carbides in a W-containing alloy steel.

BACKGROUND OF THE INVENTION

Ultra-high-strength steel is a steel material with high strength, high plasticity and good toughness that is prepared from structural alloy steel by alloying, heat treatment, and hot or cold processing. In this way, the strength of the steel is significantly improved while maintaining good plasticity and toughness, particularly with a tensile strength more than 1500 MPa (or a yield strength over 1380 MPa). As the research level improved, the ultra-high-strength steel has become an important class of materials that is widely applied to some particular fields such as landing gears and horizontal stabilizer shafts of aircrafts, bulletproof steel plates of armored vehicles, bogies of large-sized machinery, engine casings of rockets, etc. The ultra-high-strength steel extends to the applications such as high sea and deep sea platforms, military equipments, mining machinery, vehicle engineering and construction engineering. The new generation of high-strength structural steels requires both ultra-high strength and super-toughness, and this is going to be a great challenge to the development of steels.

The secondary hardening of ultra-high-strength steel is characterized by a quenching-tempering (Q-T), a quenching-partitioning (Q-P) or a quenching-partitioning-tempering (Q-P-T) process. A precipitation-hardening effect is generated by dispersed alloy carbides, resulting in significantly enhanced strength and hardness as well as improved toughness. However, the content of alloying elements such as precious metals, for example Ni and Co in the material is very high, leading to high production cost.

The ultra-high-strength steels of precipitation hardening have ultra-high strength, excellent plasticity and toughness, and are widely used in the fields of armor steels, bulletproof steel plates, bearing steels and so on. Scientists and engineers have done a lot of research and development work. For example, Chinese Patent Application No. CN108004475A disclosed a “900 MPa grade hot-rolled nanoscale precipitation hardening steel with high-strength and high toughness and manufacturing method thereof”, in which a structure with a microstructure of bainite+nanoscale carbide was produced by adding precious alloying elements such as Nb, Ti, Mo, V, and then a 900 MPa grade hot-rolled nanoscale precipitation hardening steel with high-strength and high-toughness was obtained. However, its alloy composition and the heat treatment process were complicated and the cost was very high. Also, Chinese Patent Application No. CN104271787A disclosed “Precipitation hardening martensitic steel and manufacturing method thereof”. Good performances, for example increased strength, were imparted to the steel through the precipitation of second phase particles by adding high content of alloying elements such as Ni (7.5˜11.0%), Cr (10.5˜13.5%), Mo (1.75˜2.5%), which however increases the cost and lowers the production efficiency because the heat treatment would require several h.

SUMMARY OF THE INVENTION

To overcome the shortcomings in the prior art, the present invention provides a heat treatment method for accelerating precipitation of nanoscale carbides in a W-containing alloy steel with a simple process, low cost and a shortened production cycle, which can improve the strength of W-containing alloy steels.

The heat treatment method for accelerating precipitation of nanoscale carbides in a W-containing alloy steel comprises: firstly, homogenizing a billet at 1100-1200° C. for 24-48 h followed by furnace cooling to room temperature; then, austenitizing the billet at 850-1000° C. for 20-40 minutes followed by quenching in iced brine; and finally heating the billet to 650-750° C. with a rate of 3-7° C./min under a vacuum of 10⁻³-10⁻² Pa and a magnetic field of 10-14 T and isothermalizing the billet for 0.5-3.0 h followed by furnace cooling to room temperature.

The chemical composition of the billet is 0.06-0.14 wt % C, 1.50-3.00 wt % W, <0.01 wt % P, <0.005 wt % S, Fe and an inevitable impurity.

A temperature of the iced brine is −3 to −1° C.

The present invention has the following advantages as compared to the prior art.

1. In the heat treatment method of the invention, the alloying elements are completely diffused after the billet is homogenized at a high temperature, and the banded structure during hot-rolling is eliminated to avoid a non-uniform structure. The billet is austenitized and water-quenched to obtain a lath martensite followed by a tempering heat treatment in a strong magnetic field. Such heat treatment leads to a simple process, shortened production cycle and improved production efficiency.

2. In the present invention, only the alloying element W is added which greatly reduces the cost compared with other patents or applications in which complex and expensive alloying elements are employed for the production of dispersed carbides. Further, low-carbon and high-tungsten alloys are used to achieve high-temperature creep strength under the strong magnetic field. Alloy carbides such as M₇C₃, M₂₃C₆, M₆C are important constituent phases in heat-resistant steels, and their precipitation and evolution have an extremely important influence on the high-temperature creep properties of heat-resistant steels, such as low-activated steels. M₂₃C₆ carbide tends to be coarsened at a grain boundary under a high-temperature service. Reduced C content can lower the coarsening of M₂₃C₆ at the grain boundary, and increased W content can stabilize the M₂₃C₆ at the grain boundary and improve the solid solution strengthening, thus improving the strength of the W-containing alloy steel.

3. In the present invention, a high magnetic field is applied during the tempering process of W-containing steel, and a structure having a large amount of dispersed nanoscale particles with ferrite as a matrix is manufactured. The application of the high magnetic field increases the driving force of phase transformation, so that the nucleation barrier of carbide is lowered and the nucleation rate of the carbide is increased. Without the application of magnetic field during tempering, the surface density of the precipitation is about 22.9%, and the particle size is 5-15 nm. When the magnetic field is applied, the particle size is substantially unchanged, but the surface density of the particles increases to 31.6%, which is about 39% higher than that of the particles when no magnetic field is applied. It can be seen that the high magnetic field significantly accelerates the precipitation of nanoscale carbides in the W-containing steel during the tempering process, enhancing the precipitation hardening and thus the strength of W-containing alloy steel.

Therefore, the heat treatment method according to the present invention is simple and has a low-cost process and a short production cycle, which apparently improves the strength of the W-containing alloy steel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission electron micrograph showing the precipitation of nanoscale carbides in a W-containing alloy steel by a heat treatment method according to the present invention.

FIG. 2 is a transmission electron micrograph showing the precipitation of nanoscale carbides in a W-containing alloy steel by the method in FIG. 1 when no magnetic field is applied.

FIG. 3 is a transmission electron micrograph showing the precipitation of nanoscale carbides in a W-containing alloy steel by another heat treatment method according to the present invention.

FIG. 4 is a transmission electron micrograph showing the precipitation of nanoscale carbides in a W-containing alloy steel by the method in FIG. 3 when no magnetic field is applied.

DETAILED DESCRIPTION OF EMBODIMENTS

The invention is further described below in conjunction with the drawings and embodiments, which are not intended to limit the scope of the invention.

Example 1

This embodiment provides a heat treatment method for accelerating the precipitation of nanoscale carbides in a W-containing alloy steel.

The billet was homogenized at 1100-1150° C. for 36-48 h followed by furnace cooling to room temperature. The billet was austenitized at 900-1000° C. for 20-30 minutes and quenched in iced brine with a temperature of −3 to −1° C. Then, the billet was heated to 700-750° C. with a rate of 3-7° C./min under a vacuum of 10⁻³ to 10⁻² Pa and a magnetic field of 10-13 T. The billet was isothermally held for 0.5-2.5 h, and then was cooled to room temperature.

The chemical composition of the billet is 0.06-0.12 wt % C, 1.50-2.50 wt % W, <0.01 wt % P, <0.005 wt % S, Fe and inevitable impurities.

FIG. 1 is a transmission electron micrograph showing precipitation of nanoscale carbides in a W-containing alloy steel by the heat treatment method according to the present invention. FIG. 2 is a transmission electron micrograph showing the precipitation of nanoscale carbides in a W-containing alloy steel by the method in FIG. 1 when no magnetic field is applied.

As shown in FIGS. 1 and 2, the nanoscale particles are M₆C carbides. It can be seen from the comparison between FIGS. 1 and 2 that the amount of nanoscale carbides is significantly increased after the product is subjected to the tempering heat treatment with a 12 T magnetic field. Compared with the heat treatment without magnetic field, the surface density of carbides is increased by about 39%, indicating that the application of high magnetic fields significantly increases the nucleation rate of the carbide, which enhances the precipitation hardening and thus the strength of W-containing alloy steel.

Example 2

This embodiment provides a heat treatment method for accelerating precipitation of nanoscale carbides in a W-containing alloy steel. First, the billet was homogenized at 1150-1200° C. for 24-36 h followed by furnace cooling to room temperature. Then, the billet was austenitized at 850-950° C. for 30-40 minutes, and was quenched in iced brine with a temperature of −3 to −1° C. Finally, the billet was heated to 650-700° C. with a rate of 3-7° C./min under a vacuum of 10⁻³-10⁻² Pa and a magnetic field of 11-14 T and was isothermally held for 1.0-3.0 h, then was cooled to room temperature.

The chemical composition of the billet is 0.08-0.14 wt % C, 2.00-3.00 wt % W, <0.01 wt % P, <0.005 wt % S, Fe and inevitable impurities.

FIG. 3 is a transmission electron micrograph showing precipitation of nanoscale carbides in a W-containing alloy steel by another heat treatment method according to the present invention. FIG. 4 is a transmission electron micrograph showing the precipitation of nanoscale carbides in a W-containing alloy steel by the method of FIG. 3 when no magnetic field is applied.

As shown in FIGS. 3 and 4, the nanoscale particles are M₆C carbides. It can be seen from the comparison between FIGS. 3 and 4 that the amount of nanoscale carbides is significantly increased after the product is subjected to the tempering heat treatment with a 12 T magnetic field. Compared with the heat treatment without magnetic field, the surface density of carbides is increased by about 55%, indicating that the application of strong magnetic fields significantly increases the nucleation rate of the carbide, which enhances the precipitation hardening and thus the strength of W-containing alloy steel.

The present invention has the following advantages as compared to the prior art.

1. In the heat treatment method of the invention, the alloying elements are completely diffused after the billet is homogenized at a high temperature, and the banded structure during hot-rolling is eliminated to avoid a non-uniform structure. The billet is austenitized and water-quenched to obtain a lath martensite followed by a tempering heat treatment in a high magnetic field. Such heat treatments lead to a simple process, shortened production cycle and improved production efficiency.

2. In the present invention, only the alloying element W is added which greatly reduces the cost compared with other patents or applications in which complex and expensive alloying elements are employed for the production of dispersed carbides. Further, low-carbon and high-tungsten alloys are used to achieve high-temperature creep strength under the strong magnetic field. Alloy carbides such as M₇C₃, M₂₃C₆, M₆C are important constituent phases in heat-resistant steels, and their precipitation and evolution have an extremely important influence on the high-temperature creep properties of heat-resistant steels, such as low-activated steels. M₂₃C₆ carbide tends to be coarsened at a grain boundary under a high-temperature service. Reduced C content can lower the coarsening of M₂₃C₆ at the grain boundary, and increased W content can stabilize the M₂₃C₆ at the grain boundary and improve the solid solution strengthening, thus improving the strength of the W-containing alloy steel.

3. In the present invention, a high magnetic field is applied during the tempering process of W-containing steel, and a structure having a large amount of dispersed nanoscale particles with ferrite as a matrix is manufactured. The application of the high magnetic field increases the driving force of phase transformation, so that the nucleation barrier of carbide is lowered and the nucleation rate of the carbide is increased. Without the application of magnetic field during tempering, the surface density of the precipitation is about 22.9%, and the carbides' size is 5-15 nm. When the magnetic field is applied, the carbides' size is substantially unchanged, but the surface density of the carbides increases to 31.6%, which is about 39% higher than that of the carbides when no magnetic field is applied. It can be seen that the high magnetic field significantly accelerates the precipitation of nanoscale carbides in the W-containing steel during the tempering process, enhancing the precipitation hardening and thus the strength of W-containing alloy steel.

Therefore, the heat treatment method according to the present invention is simple and has a low-cost process and a short production cycle, which apparently improves the strength of the W-containing alloy steel. 

What is claimed is:
 1. A heat treatment method for accelerating precipitation of nanoscale carbides in a W-containing alloy steel, comprising: homogenizing a billet at 1100-1200° C. for 24-48 h followed by furnace cooling to a room temperature; austenitizing the billet at 850-1000° C. for 20-40 minutes followed by quenching in iced brine; and heating the billet to 650-750° C. with a rate of 3-7° C./min under a vacuum of 10⁻³-10⁻² Pa and a magnetic field of 10-14 T, isothermalizing the billet for 0.5˜3.0 h and then cooling the billet to room temperature; wherein a chemical composition of the billet is 0.06-0.14 wt % C, 1.50-3.00 wt % W, <0.01 wt % P, <0.005 wt % S, Fe and an inevitable impurity.
 2. The method of claim 1, wherein a temperature of the iced brine is −3 to −1° C. 